The present invention relates to a method for correction of the spark advance for an internal combustion engine with a continuous phase transformer at the intake and/or exhaust.
As is known, in an endothermic engine, the optimum spark advance depends on the speed of rotation and on the engine load (which, in systems with combustion with a stoichiometric mixture ratio, depends on the mass of air collected during the intake phase). Combustion primed with optimum spark advance corresponds to the maximum torque generation (excluding the occurrence of detonating phenomena), whereas combustion primed with any other advance corresponds to deterioration of the combustion performance.
In addition to the mass of air collected during the intake phase, the quantity of residual burnt gas (which is trapped inside the cylinder at the end of the exhaust phase, and is altogether inert as far as combustion is concerned) plays a determining part in definition of the optimum advance, since both the quantity and quality of the fuel present in the cylinder at the end of the intake phase affect the speed of propagation of the flame front and the combustion temperature. In particular, the speed of propagation of the flame front (which affects in a determining manner the spark advance applicable) for the same new mass (of air) collected, is reduced as the percentage of burnt gas present in the combustion chamber increases.
The presence of a continuous phase transformer at the intake/exhaust modifies both the quantity and quality of the fuel collected, and thus the speed of propagation of the flame front. In fact, in engines which have intake/exhaust timing devices with continuous transformation, for the same engine rotation speed it is possible to obtain the same quantity of mass of air collected in the cylinder, with different intake/exhaust timing values, and the different intake/exhaust timing values correspond to a different mass of inert gas which is trapped in the cylinder at the end of the exhaust phase.
From the foregoing information it is apparent that in order to determine a correct value of the advance to be applied, it is necessary to take into account also the mass of inert gas which is trapped in the cylinder at the end of the intake phase. In the known internal combustion engines, use is made of mapping (or dimensioned plans), which are determined in the design phase, are stored in a memory of a control unit, and can supply the correct value of the advance according to the drive point (which is defined by the speed of rotation and by the engine load), and according to the intake/exhaust timings (which are contained within their minimum and maximum values).
However, the use of the aforementioned mapping is particularly costly, since, in order to be stored in the control unit of the engine, this mapping requires a quantity of memory which is relatively very high. In addition, the logic used for this mapping is completely different from the logic implemented by the control unit of engines with an external EGR circuit (i.e. with an external fuel gas recirculation circuit), and therefore the development of this mapping cannot borrow in any way from the knowledge developed during the design of the engine control logic with an external EGR circuit, a fact which is clearly a requirement for more time during the design and adjustment stage.
The object of the present invention is to provide a method for correction of the spark advance for an internal combustion engine with a continuous phase transformer at the intake and/or exhaust, which is free from the above-described disadvantages, and in particular is easy and economical to implement.
According to the present invention, a method is provided for correction of the spark advance for an internal combustion engine with a continuous phase transformer at the intake and/or exhaust, as described in claim 1.